The evolution of cellular communications has resulted in a proliferation of networks of different technologies and corresponding different air interfaces. As a result, during the course of a single voice call, a wireless subscriber unit may roam among multiple networks, wherein each such network implements a different technology than the other networks of the multiple networks. Among the different network technologies are packet switched CDMA (Code Division Multiple Access) technologies, such as CDMA 2000 1XEV-DO (1X Evolution Data Only) or packet switched CDMA 1XRTT (1X Radio Transmission Technology), that are capable of providing Voice over Internet Protocol (VoIP) communication services, and conventional, or legacy, CDMA cellular communication technologies, such as a CDMA 1X, that provide circuit switched voice communication systems.
As the subscriber unit roams among a packet switched CDMA communication network and a circuit switched CDMA communication network, it may be beneficial to system performance to handoff the subscriber unit from the former network to the latter system performance to handoff the subscriber unit from the former network to the latter network or from the latter network to the former network. For example, the channel conditions associated with one such network may be more favorable than the channel conditions associated with the other such network due to such factors as fading, adjacent and co-channel interference, and available power at a serving base station (BS) or radio access network (RAN). By way of another example, an operator of both a packet switched CDMA network and a circuit switched CDMA network may desire to move the subscriber unit from one such network to the other such network for purposes of balancing system loading.
Currently, the only defined method for executing a handoff between a packet switched CDMA or WLAN network and a circuit switched CDMA network is an execution of a hard handoff, wherein a subscriber unit must drop a radio resource of a network of a first CDMA technology prior to acquiring a radio resource of a network of a second CDMA technology. A result is a brief period of time during which the subscriber unit is not actively engaged in a communication session with either network. Further, when executing a hard handoff there is no linkage between the two networks as the subscriber unit must drop the first network and acquire the second network without any assistance from the BS or RAN of either network. As a result, voice traffic may be lost during the handoff, resulting in poor system performance and efficiency and disgruntled end users.
Therefore, a need exists for a method and apparatus for an active handoff of a voice call between a packet switched CDMA or WLAN network and a circuit switched CDMA network that assures that the subscriber unit is actively engaged in a communication session with at least one network at all times.